Wheatstone bridge circuits are utilizing in a variety of sensing applications, particularly those involving magneto-resistance, which is a phenomenon whereby the electrical resistance of a body can be influenced by magnetic flux. In such applications, the electrical resistance of the body changes in a predictable manner in response to a varying magnetic flux, making such a body suitable for use as a magnetic-electric transducer in a magnetic field sensor. However, as with any resistive body, the electrical resistance of such a body can also be influenced by other environmental factors, such as, for example, temperature. A problem in practical, but sensitive sensor applications involves configuring some means of differentiating between transducer signals resulting from varying magnetic flux and unwanted transducer signals emanating from other environmental sources. A popular approach is to include at least one magneto-resistive element in a Wheatstone bridge arrangement.
It is known to measure many physical parameters by using a Wheatstone bridge measuring circuit. Typically, the Wheatstone bridge measuring circuit includes two parallel branches, each of which includes two series arms. In one of the arms, parameter-responsive impedance, such as a temperature responsive resistor, can be connected. To determine the value of the variable impedance, and therefore the value of the parameter being monitored, the prior art bridges are generally adjusted to be in a balanced state, whereby a null voltage is developed across a diagonal of the bridge. The diagonal is between taps on the two branches. The bridge can be activated to a balanced condition either manually or automatically, by adjusting values of impedances, other than the monitoring impedance, of the bridge.
After balance has been achieved, the value of the variable parameter responsive impedance is calculated by using the well known balance equation. From the calculated value of the parameter responsive impedance, the value of the parameter is calculated from a known relationship between the parameter value and the impedance value of the parameter responsive impedance. These calculations can be performed either manually or automatically.
Thus, many sensors utilize a Wheatstone bridge circuit configuration, which is essentially a very simple circuit. Compensation of the sensor, however, requires the addition of resistances in series or parallel with the active components. Recently, programmable compensation integrated circuits (IC's) have been implemented, which allow digital compensation of the sensor circuitry. Due to the limitations of analog-to-digital and digital-to-analog converters, however, such configurations produce an amplified voltage output, which is typically in the range of 0.1 Volts to 4.9 Volts. Perfect rail-to-rail circuits, for example, are difficult to implement. Unfortunately, there are presently no IC's available, which can produce an output similar to that of an unamplified Wheatstone bridge circuit.